1. Field of the Invention
This invention relates to an apparatus for measuring the physical properties of internally and externally threaded parts, and more specifically, to an apparatus which measures the threaded part by comparing the threaded part against a known standard and displaying the difference between the part and the standard.
2. Description of the Related Art
Screws and bolts are routinely tested for function and pitch to determine whether the screws are within tolerance limits in a specification. Due to wear of cutting tools, these measurements during the manufacturing process can change over a period of time. Thus, screws and bolts must be continually tested to ensure that the screws and bolts conform to specifications. Typically, screw thread measuring devices measure only two parameters: the pitch diameter and the functional diameter of the parts. These measurements provide the best indication of whether or not the part is within the desired tolerance. The pitch diameter is defined as the diameter of the pitch cylinder for a straight thread. For a taper thread, the pitch diameter, at a given position on the thread axis, is the diameter of the pitch cone at that position. For a single-start thread of perfect form and lead, it is also the length between intercepts of a line which is perpendicular to the thread axis and intersects thread flanks on opposite sides of the thread axis. The functional diameter of an internal or external thread is the pitch diameter of the enveloping thread of perfect pitch, lead, and flank angles, having full depth of engagement but clear at crests and roots, and of a specified length of engagement. It may be derived by adding to the pitch diameter in the case of an external thread, or subtracting from the pitch diameter in the case of an internal thread, the cumulative effects of deviations from specified profile, including variations in lead and flank angle over a specified length of engagement. The effects of taper, out-of-roundness, and surface defects may be positive or negative on either external or internal threads.
Generally, there are two ways to measure the physical parameters. First, a threaded part is compared to a standard and the difference between the standard and the part appears on a display. Second, this measurement is compared to the required dimensions. When measuring an externally threaded part, the part is generally placed between two contacts which are biased against the part. The displacement of the contacts as they are pressed against the part causes a displacement in a mechanical or electrical gage, resulting in a read-out on the dial of a mechanical gage or the display of an electrical gage which represents the difference between the part and the standards. If the difference is within the desired tolerance, the threaded part is acceptable and no modification is made to the manufacturing process.
Heretofore, there were separate devices which measure only one physical parameter either the pitch diameter or functional diameter for internal or external threads. The disadvantage of these devices is that separate devices for each type of measurement for internally and externally threaded parts are required. Four separate devices are required if it is desired to measure the pitch diameter and functional diameter for both internally and externally threaded parts. This requirement significantly increases the cost by requiring the purchase of four separate devices.
U.S. Patent Nos. 3,844,046 and 3,879,854 disclose apparatuses which measure the pitch diameter and functional diameter, but only for externally threaded parts. These thread measuring devices increase cost and have limited versatility because they are incapable of measuring both the pitch diameter and functional diameter of both internally and externally threaded parts on a single thread measuring device, resulting in the need for separate thread measuring devices to measure pitch diameter and functional diameter for both internal and external threads.
The U.S. patent to Beaupere, U.S. Pat. No.4,586,261, discloses a gage capable of measuring both internally and externally threaded parts. However, Beaupere, requires switching components to measure the pitch diameter or functional diameter which reduces operating convenience as the pitch diameter and functional diameter are often used in combination to determine if the threaded part is within a desired tolerance.